Loss of sporulation phenotype in Epulopiscium
Epulopiscium is a firmicute that inhabit the gastrointestinal tract of different surgeonfish. These bacteria are unique in that they can grow to several hundred microns in length, and tens of microns in diameter. Not only are the epulopiscium large in size, they also contain several hundreds of thousands of copies of their genome that localizes to near the membrane, while delivering a single copy to the offspring . Interestingly, while these mega bacteria are phylogenically far removed from the spore-forming B. Subtilis, some of the genes that are invovled in B. Subtilis sporulation are also somewhat conserved in Epulopiscium genome. How are B. Subtilis and Epulopiscium related? One of the initial studies that looked at Epulopiscium division was looking at the localization of a B. Subtilis endospore formation called FtsZ. Through draft sequencing, the group has also identified Some proteins in Epulopiscium that share homology with other B. Subtilis sporulation. Partial Epulopiscium genome sequencing results Due to technical difficulties (such as difficulty to culture the bacteria ex vivo) sequencing could only be done to draft quality. The fact that Epulopiscium species shares only a distant relation to B. Subtilis made the direct comparison (via sequencing) more difficult. It could be that there are genes that are not highly conserved that may result in similar functions in Epulopiscium. However, they have found some interesting homology with sequences for proteins that are involved in endospore formation in B. Subtilis that could provide some insight as to when the Epulopiscium diverged from the firmicute phylum. The master regulator of sporulation, Spo0A is conserved in Epulopiscium. Normal endospore forming bacteria enter sporulation under starvation, or in other words, when nutrients are not abundant. Spores are highly resistant to environmental insults, such as UV irradiation, acid/base exposure, heat/cold exposure, etc. So one could imagine that expression of Spo0A is conditionally regulated. However, Epulopiscium seem to not depend on the nutrient availability or other environmental cues as the initiating factor of offspring formation that is not binary fission. This suggests that this non-binary fission offspring formation is the main mode of cell division for Epulopiscium. In B. Subtilits, sequential activation of certain sigma factors help the process of sporulation along to produce mature spores, and regulate mother cell lysis. One way in which the sigma factors help decide the fate of the different cell type is for compartmentalized expression of certain sigma factors at different times of endospore formation. These sigma factors, along with other core sporulation factors are conserved in Epulopiscium and another distantly related species, clostridium. Genes involved in engulfment have also been shown to share homology across the species examined in this study. Regardless of the fate of the compartmentalization (either forespore formation, or offspring formation) this process of mother cell membrane surrounding the newly partitioned forespore is conserved in both B. Subtilis and in Epulopiscium. Perhaps this is the reason why the engulfment genes are highly conserved between the two otherwise distantly related species. Late sporulation proteins were not as conserved, or they were missing from the Epulopiscium genome. This is most likely due to the fact that the late sporulation proteins are mostly invovled with the formation of the anhydrous cortex, and the proteinaceous coat that would help the spores to be resistant to environmental insults. As these bacteria give rise to offspring that do not lie dormant, the genes required for forming a protective layer were probably not necessary. References Miller DA, Suen G, Clements KD, & Angert ER (2012) The genomic basis for the evolution of a novel form of cellular reproduction in the bacterium Epulopiscium. BMC genomics 13:265. Mendell JE, Clements KD, Choat JH, & Angert ER (2008) Extreme polyploidy in a large bacterium. Proceedings of the National Academy of Sciences of the United States of America 105(18):6730-6734.